An air-conditioning system for motor vehicles is provided with various kinds of heat exchangers, e.g., a double pipe type heat exchanger. As shown in FIGS. 1 and 2, a conventional double pipe type heat exchanger includes an inner pipe 10 and an outer pipe 20. The inner pipe 10 is provided with a first flow path 12 through which a first fluid flows. The outer pipe 20 is arranged outside the inner pipe 10 so that a second flow path 30 can be defined between the outer circumferential surface of the inner pipe 10 and the inner circumferential surface of the outer pipe 20.
A second fluid flows through the second flow path 30 between the inner pipe 10 and the outer pipe 20. The second fluid flowing through the second flow path 30 differs in temperature from the first fluid flowing through the first flow path 12. Accordingly, a heat exchange action occurs between the first fluid and the second fluid when the second fluid makes contact with the first fluid.
With the double pipe type heat exchanger mentioned above, the first fluid and the second fluid differing in temperature from each other are respectively introduced into the first flow path 12 and the second flow path 30 and brought into indirect contact with each other. This enables a heat exchange action to occur between the first fluid flowing through the first flow path 12 and the second fluid flowing through the second flow path 30.
However, the conventional double pipe type heat exchanger has a drawback in that a gap G is generated between the inner pipe 10 and the outer pipe 20 due to the assembling tolerance. This may reduce the heat exchange efficiency and may cause the inner pipe 10 and the outer pipe 20 to make frictional contact with each other.
In other words, with a view to assure smooth assembling of the inner pipe 10 and the outer pipe 20, the double pipe type heat exchanger is designed such that the inner diameter L1 of the outer pipe 20 is greater than the outer diameter L2 of the inner pipe 10. Thus, an assembling tolerance exists between the inner pipe 10 and the outer pipe 20.
The assembling tolerance may become a cause of generating a gap G between the inner pipe 10 and the outer pipe 20. The existence of this gap G poses a problem in that the second fluid introduced into the second flow path flows along a straight line. This tends to sharply reduce the heat exchange time between the first fluid flowing through the first flow path 12 and the second fluid flowing through the second flow path 30. The reduction of the heat exchange time between the first fluid and the second fluid leads to a remarkable reduction of the heat exchange efficiency, which in turn significantly reduce the performance of the heat exchanger.
Another problem of the conventional double pipe type heat exchanger resides in that the gas G existing between the inner pipe 10 and the outer pipe 20 allows the inner pipe 10 to move within the outer pipe 20. Thus, the inner pipe 10 is likely to make contact with the inner circumferential surface of the outer pipe 20.
In particular, if the vibration of a motor vehicle is transferred to the inner pipe 10, the inner pipe 10 vibrates at a high speed. This causes the inner pipe 10 and the outer pipe 20 to make frictional contact with each other. As a result, contact noises may be generated between the inner pipe 10 and the outer pipe 20, and the contact portions of the inner pipe 10 and the outer pipe 20 may be worn. The contact wear of the inner pipe 10 and the outer pipe 20 may significantly reduce the durability of the heat exchanger, thereby shortening the lifespan of the heat exchanger.